The present invention relates to an air/fuel (A/F) ratio sensor for detecting the A/F ratio of an air/fuel mixture being supplied into a combustor. More particularly, the present invention relates to an A/F ratio sensor that is capable of detecting the A/F ratio of an air/fuel mixture using an oxygen ion conductive solid electrolyte over the full operating range, including the lean region (where air is in excess of the stoichiometric value) to the rich region (where fuel is in excess of the stoichiometric value).
With a view to improving fuel economy and reducing emissions, some conventional combustors such as internal combustion engines are provided with the capability of feedback control, involving the detection of oxygen levels in the exhaust gas and control of the air/fuel mixture in the combustion chamber so as to burn it at an A/F ratio in the vicinity of the stoichiometric value. An A/F ratio sensor commonly used to detect the concentration of oxygen in the exhaust gas takes the form of an oxygen sensor that is based on a transition metal oxide such as TiO.sub.2 or CoO, the resistance of which varies depending upon the partial pressure of oxygen gas in the gas to be analyzed. Generally, this type of A/F ratio sensor produces an output voltage that changes abruptly at the stoichiometric value of the A/F ratio of the air/fuel mixture.
Attempts are constantly being made to maximize the performance of combustors, in addition to providing fuel economy improvements and emissions reduction, by means of performing feedback control to attain a desired A/F ratio that is adaptive to a specific state of operation of the combustor. This goal, however, is not attainable by the aforementioned oxygen sensor which is merely capable of detecting the stoichiometric value of the A/F ratio of the air/fuel mixture.